The present invention relates to a feed unit, in particular for the feeding of fuel, having an electric drive motor and a pump unit coupled to it, the pump unit comprising a base plate provided with an inlet opening, an intermediate plate which rests against the base plate and has a recess in which rotating pump parts made of sintered metal, in particular a gerotor, are mounted for rotation, and a cover which rests on the intermediate plate and has an outlet opening as well as a passage opening for the shaft of the drive motor.
Feed units of this type are already known in which the base plate, the intermediate plate and the cover consist of a sintered steel. The rotating pump parts are arranged for rotation in the recess of the intermediate plate, and are constructed also of a sintered steel. They may be a plurality of rollers if the pump unit is developed as a roller vane pump or a pair of gear wheels which comprises an internally toothed and an externally toothed gear, a so-called internal geared wheel pump or else two externally toothed gears of an ordinary gear pump. In order to keep the wear between the rotating pump parts and the stationery pump parts slight, the base plate, the intermediate plate and the cover are subjected to an oxidizing vapor treatment whereby a hard oxide skin is formed on these parts, thus considerably reducing the wear between the different pump parts.
In order to obtain the highest possible efficiency of the pump, the smallest possible axial spaces are desired between the rotating pump parts and the cover or the base plate. Roughnesses of up to 5 .mu.m can be obtained by suitable treatment of the sintered metal parts. However, due to the required vapor treatment, deformations on the order of magnitude of 8 to 10 .mu.m are produced in the material treated in this manner. Since the oxide layer produced by the vapor treatment is only about 5 to 6 .mu.m thick, even a reworking of the parts subjected to the vapor treatment results in only a slight improvement of the roughnesses.
Another disadvantage of these known pumps resides in the fact that their efficiency declines with age, due to the wear which occurs even with a pair of materials such as sintered steel and sintered steel oxide.